The present invention relates to a developing unit for use in an image forming apparatus, such as a printer, a facsimile machine or a copying machine, which forms an image by using an electrophotography technique.
In general, an image forming apparatus using an electrophotography technique includes a photosensitive member having a photosensitive layer on the outer surface thereof, a charging device for uniformly electrically charging the outer surface of the photosensitive member, an exposing device for selectively exposing the outer surface, which has been uniformly electrostatically charged by the charging device, so as to form an electrostatic latent image, a developing device for supplying toner serving as a developer to the electrostatic latent image formed by the exposing device so as to form a visible image (a toner image) and a transfer device for transferring the toner image formed by the developing device to a transfer medium, such as paper.
A conventional developing unit includes a case for accommodating toner; a developing roller rotatably supported by the case by dint of a shaft; a supply roller rotatably supported by the case by dint of a shaft and formed of an elastic member arranged to be pressed against the surface of the developing roller so as to supply toner to the surface of the developing roller; and a conveying fin rotatably supported by the case by dint of a shaft in such a manner as to convey toner to the surface of the supply roller, wherein the elements are sequentially disposed in a horizontal direction.
A conventional developing unit also includes a restraining blade for restraining a quantity of toner on the developing roller, as well as a sealing member disposed on the sides of the developing roller and the supply roller so as to prevent leakage of toner through their shaft portions to the outside of the case.
The conventional developing unit uses a known contact development method in which the developing roller and the photosensitive member are disposed adjacently or in contact with each other. In the contact method, an edge of the end portion of the surface of the developing roller for conveying toner comes in direct contact with the photosensitive member, which can undesirably damage the photosensitive member. If the developing roller is made of a solid material, such as metal, even a non- contact development method encounters damage of the photosensitive member attributable to sliding and friction depending upon the accuracy in the deflection of the developing roller and that of the photosensitive member, as well as the contact development method.
Further, the conventional developing unit, has an elastic conveying member made of an expanded material or the like. Therefore, toner is continuously conveyed from the conveying member to the supply roller portion. As a result, the quantity of toner which is conveyed is larger than the quantity of toner which has been consumed by the developing roller in forming images. Thus, toner is compressed in the supply roller portion and the developing roller portion. If such compression is continued, the pressure in the case at the positions near the developing roller is excessively raised by the restraining blade to appropriately restrain toner on the developing roller. The excessive conveyance of toner from the restraining blade changes the density of a -formed image and causes toner to be leaked. If color toner having unsatisfactory fluidity as compared with that of black toner is used, the above-mentioned compression becomes more critical because color toner having poor fluidity is continuously conveyed in the developing unit, and excess toner cannot be returned from the supply roller.
If the conventional developing unit performs a development process with dense toner having poor fluidity, the conveying fin generates a great rotational load which undesirably changes the necessary torque thereby causing rotation of the motor which drives the conveying fin. As a result, jitters appear in the formed image.
Moreover, the conventional developing unit suffers the problem that filming of the electrified members (such as the developing roller and the restraining blade), easily occurs because of mechanical contact and friction when the development process is performed using dense toner containing a large quantity of pigment. If filming of, for example, the pigment having the same polarity as that of toner, occurs with the electrified member, the electrification characteristic of toner deteriorates and becomes instable. When the electrification characteristic of the toner is instable, the density of the formed image is lowered and the toner supply characteristic deteriorates.
The conventional developing unit also has the toner seal disposed on the outer surface ends of the developing roller, thus toner in the sealing portion of the developing roller is not covered. Therefore, in the known contact development method, the photosensitive member is damaged because the sealing portion directly slides on the photosensitive member. When the developing roller is made of a solid material, such as metal, even a non-contact development method encounters damage of the photosensitive member attributable to sliding and abrasion depending upon the accuracy in the deflection of the developing roller and that of the photosensitive member, as well as the contact development method.
One problem encountered by the conventional developing unit is that the portion of the elastic supply roller pressed against the developing roller is dented, causing end portions of the supply roller to project sideways. The projecting portions then undesirably engage the sealing member, thereby requiring an excessively large drive torque in order to rotate the supply roller.
Another problem with the foregoing conventional developing unit arises from the hardening of the supply roller. The supply roller of the foregoing conventional developing unit is comprised of an elastic member, which is usually an expanded material having cells formed in the surface thereof. The portion of the supply roller which contacts with the developing roller encounters introduction of toner into the expanded material through the cells formed in the surface of the expanded material. As a result, the hardness of the expanded material is increased excessively after being used for a long time. The problem associated with the increase in hardness of the supply roller is that great torque is required to rotate the supply roller. To solve this problem, a supply roller comprising a closed-cell expanded material has been suggested. However, in recent years, the average particle size of toner has been reduced to 9 .mu.m, and toner having such small particle size easily clogs in the cells formed in the surface of the conventional supply roller. The elastic characteristic of the supply roller thus deteriorates in a relatively short time.
Another problem in the foregoing conventional developing unit is that, if the rotational speed of the developing roller is increased to quickly form images, or if the fluidity of toner is increased to maintain the required toner supply characteristic, then toner is introduced into the end surface (the side surface) of the developing roller when the developing roller is rotated. As a result, toner leaks from the end surface of the developing roller into the image forming portion thus causing the inside portion of the image forming apparatus to be contaminated. Another problem associated with increased rotational speed of the developing roller and increased fluidity of toner is the leakage of toner from the lower surface of the developing roller during rotation of the developing roller. This also contaminates the inside portion of the image forming apparatus. When the image forming apparatus is contaminated in either above manner, it produces a defective image.
Another problem in the above-mentioned conventional developing unit is that an edge of the end portion of the surface of the developing roller comes in direct contact with the photosensitive member thereby causing damage to the photosensitive member.
The photosensitive member is also damaged by direct sliding contact with the sealing portion of the developing roller when a contact development method is used. In a contact development method, the developing roller and the photosensitive member are disposed adjacently or are brought into contact with each other. The toner seal for the developing roller of the conventional developing unit is disposed at the outer surface of the ends of the developing roller which leaves toner in the sealing portion of the developing roller not covered and which allows the seal to contact the photosensitive member.
In either case, if the developing roller is made of a solid material, such as metal, even a non-contact development method causes damage to the photosensitive member attributable to sliding and friction depending upon the accuracy in the deflection of the developing roller and that of the photosensitive member.
Another problem in the above-mentioned conventional developing unit arises in the conveyance of toner. Toner in the case is sequentially conveyed by the conveying member to the supply roller portion, and then from the supply roller to the developing roller portion. In a developing unit having an elastic conveying member made of an expanded material or the like, conveyance of toner from the conveying member to the supply roller portion is continuous. As a result, the quantity of toner which is continuously conveyed is larger than the quantity of toner which has been consumed for forming images by the developing roller. The excess toner in the vicinities of the supply roller portion and the developing roller portion causes a state of compression. If the foregoing compression is continued, pressure in the case is raised at positions near the developing roller. As a result, toner on the developing roller cannot be restrained by the restraining blade, which leads to an excess conveyance of toner that causes undesirable changes in the density of the image and also causes undesirable toner leaks.
The above problems are exacerbated when color toner is used. Color toner generally has fluidity inferior to that of black toner. Specifically, color toner contains resin of a type having a multiplicity of low-molecular-weight components in order to realize color transmissivity and a dispersant for uniformly dispersing color pigment. The foregoing components deteriorate the fluidity of the toner. If color toner having poor fluidity is continuously conveyed in the above-mentioned developing unit, excess toner cannot be returned from the supply roller, which makes the state of compression more critical.
Additional problems with the conventional developing unit arise when color toner is used. The foregoing electrophotographic process using color toner is performed in such a manner that four developing units, for forming yellow, magenta, cyan and black images, are disposed in the apparatus. The use of four developing units increases the size of the apparatus so that it is much larger than an apparatus for forming a monochrome image. To decrease the size of the apparatus, the density of pigment in each toner particle must be increased to reproduce a required image density with a smaller quantity of toner. By using a smaller quantity of toner, the capacity of the toner case can be reduced.
However, if the pigment component in the toner is increased, the fluidity of the toner generally deteriorates, thus causing a great rotational load on the conveying fin. The increased load on the conveying fin undesirably changes the necessary torque to drive the conveying fin which in turn causes undesirable changes in the rotation of the motor which drives the conveying fin. As a result, jitters appear in the formed image.
Increasing the pigment component in the toner also raises the area ratio of the pigment component on the surface of the matrices of toner particles in general. Toner must have a certain polarity and be frictionally electrified by an electrified member having a polarity opposite to the polarity of toner (such as a developing roller or a restraining blade) so that the electrification of toner is stabilized.
If toner particles have pigment in a large quantity on their surfaces, the electrified members (such as the developing roller and the restraining blade) easily encounter filming attributable to mechanical contact and sliding. If the electrified member having the same polarity as that of toner encounters filming, the electrification characteristic of toner deteriorates and becomes instable. As a result the density of the formed image is lowered and the toner supply characteristic-deteriorates.
Further problems arise in the above-mentioned conventional developing unit when trying to reduce its size. The center or rotation of the conveying fin in the conventional developing unit is disposed lower than the center of rotation of the supply roller in an attempt to reduce the thickness of the developing unit by efficiently creating a space for accommodating toner. However, such an arrangement suffers the problem that the conveying fin scrapes insufficient toner up to the surface of the toner supply roller thereby causing an undesirably low density in the formed image.